Conventionally, a gear-type transmission (planetary gear type) and a continuously variable transmission (belt type or toroidal type) have been known as an automatic transmission structured to hydraulically control a transmission mechanism. The belt-type continuously variable transmission includes a driving revolution member, a driven revolution member, and a wound power transmission member wound around the driving revolution member and the driven revolution member, and its transmission ratio is controlled by hydraulic control of a winding radius of the wound power transmission member around the driving revolution member.
The belt-type continuously variable transmission includes an input shaft receiving engine torque, an output shaft provided in parallel to this input shaft, a primary pulley provided on the input shaft side, and a secondary pulley provided on the output shaft side. The primary pulley has a fixed sheave fixed to the input shaft, and a movable sheave capable of moving in an axial direction of the input shaft. The secondary pulley has a fixed sheave fixed to the output shaft, and a movable sheave capable of moving in an axial direction of the output shaft. A belt is wound around the primary pulley and the secondary pulley structured as above. Further, there are provided a first hydraulic chamber (fluid pressure chamber) for controlling operation of the movable sheave of the primary pulley and a second hydraulic chamber for controlling operation of the movable sheave of the secondary pulley. By controlling a hydraulic pressure in the first hydraulic chamber, a groove width of the primary pulley is varied, in other words, a winding radius of the belt on the primary pulley side is varied, so that a transmission ratio is controlled.
More specifically, transmission control in such a belt-type continuously variable transmission is performed, for example, by determining a target value for the transmission ratio, detecting an actual value of the transmission ratio of the continuously variable transmission, and executing feedback control with a transmission actuator in accordance with a difference between the target value and the actual value. With this feedback control, in a coasting state, a response of the transmission ratio is a ramp response where the target transmission ratio increases with time, which results in particularly poor responsiveness and following capability of control, and when a vehicle is decelerated rapidly, a speed change toward a larger transmission ratio of the continuously variable transmission is delayed, and in some cases a speed change to a maximum transmission ratio may be impossible before the vehicle stops. If a throttle is opened for reacceleration during such a speed change, frictional force does not act sufficiently on the belt because the speed change toward a larger transmission ratio is still in progress and hydraulic fluid is still being discharged, causing belt slip and failure in motive power transmission as well as belt wear.
Japanese Patent Laying-Open No. 63-43837 (hereinafter referred to as “Patent Document 1”) discloses a transmission control device for a continuously variable transmission which solves such problems. Patent Document 1 discloses increasing transmission gain to effect an immediate speed change toward a larger transmission ratio when a brake is applied in a coasting state.
A torque converter is provided between an engine and an automatic transmission, and a torque converter usually includes a lock-up clutch. The lock-up clutch mechanically couples a driving member (a pump impeller on the engine side) of the torque converter and a driven member (a turbine runner on the transmission mechanism side) directly to each other, and can thus achieve both improvement of fuel efficiency and ride comfort. A lock-up region where such a lock-up clutch is engaged is set based on a vehicle speed and an opening position of the throttle, for example.
Moreover, a technique has been known for controlling a lock-up clutch to execute feedback control (slip control) of clamping force (engagement pressure, clamping differential pressure, differential pressure) of that lock-up clutch to a prescribed state in accordance with a speed difference between a pump speed on the input side (which corresponds to an engine speed) and a turbine speed on the output side, and based on a learned value acquired at this time, controlling a slip state of the torque converter appropriately to prevent occurrence of noise and vibration (NV) and improve starting performance of the vehicle.
In this manner, distribution of motive power transmission in a mechanical manner by the lock-up clutch and motive power transmission by the torque converter is finely controlled in accordance with a running state by sophisticated electronic control, thereby significantly increasing transmission efficiency. That is, this lock-up clutch is controlled based on a driving state of the vehicle such as load, revolution and the like, and for example, a low-load and high-revolution region is set as a lock-up region, a high-load and low-revolution region is set as a converter region, and a low-load and intermediate-revolution region is set as a slip region. In the lock-up region, an input element (pump impeller) and an output element (turbine runner) of the torque converter serving as a fluid-type power transmission are completely clamped to each other to improve fuel efficiency performance. In the converter region, the input element and the output element of the fluid-type power transmission are completely disengaged from each other, and torque is increased by a torque-amplifying function of the torque converter. Further, in a coasting state, the input element and the output element of the fluid-type power transmission are half-clamped to each other in the slip region to achieve both improvement of fuel efficiency performance and absorption of shock and vibration.
If a clamping differential pressure of the lock-up clutch has been set to a maximum value (a state with slight slip and the smallest amount of slip) in this slip region, rapid deceleration in a freewheeling state causes lowering in the clamping differential pressure of the lock-up clutch from the maximum value. Thus, lowering the clamping differential pressure of the lock-up clutch to release lock-up (disengage the lock-up clutch) tends to be delayed, which may result in engine stall. In such a slip state, therefore, a borderline clamping differential pressure which barely avoids slip is set, and when large torque is input from wheels due to rapid deceleration, an amount of slip is ensured to thereby prevent engine stall. This borderline clamping differential pressure is calculated as follows. Specifically, a small clamping differential pressure which does not cause slip (hereinafter referred to as an initial differential pressure) is once provided at the start of coasting lock-up, this initial differential pressure is lowered by using PI control and the like to a clamping differential pressure where a small amount of slip is obtained, and a prescribed offset differential pressure is added to the clamping differential pressure where the small amount of slip was obtained (learned differential pressure). By using this (learned differential pressure+offset differential pressure) as a clamping differential pressure, occurrence of engine stall is prevented even during rapid deceleration while improving fuel efficiency.
In such learning and controlling, learning takes time depending on an individual difference of the lock-up clutch, which may result in engine stall if rapid deceleration occurs during this time. Japanese Patent Laying-Open No. 2004-124969 (hereinafter referred to as “Patent Document 2”) discloses a lock-up clutch control device for an automatic transmission which solves such a problem. Patent Document 2 discloses completing learning and controlling of a clamping differential pressure early in slip control of the lock-up clutch in a coasting state.
As described above, when learning and controlling of a lock-up clutch has been completed, the lock-up clutch can be controlled appropriately. That is, even when a brake is actuated in a coasting state, the lock-up clutch can be controlled appropriately to be disengaged, thereby preventing engine stall. Namely, even when a speed of a drive wheel is reduced rapidly due to hard braking, the lock-up clutch can be disengaged immediately, thereby attaining control for preventing reduction in engine speed.
When learning and controlling of a lock-up clutch has not been completed, however, the lock-up clutch may not be controlled appropriately (the lock-up clutch may not be disengaged immediately during hard braking), and hence it is controlled as follows. When a brake is actuated in a coasting state, as described above, the lock-up clutch is controlled by increasing transmission gain to effect an immediate speed change toward a larger transmission ratio to maintain the engine speed high, thereby preventing engine stall.
It is conceivable, however, that such control may result in the following situation. In order to increase the transmission gain to effect an immediate speed change toward a larger transmission ratio, a hydraulic pressure (line pressure) also used for actuating the lock-up clutch is used in large amounts for transmission control. Here, in addition to the fact that learning and controlling of the lock-up clutch is incomplete, there is a shortage of hydraulic fluid pressure (particularly when the differential pressure is set low such that the lock-up clutch can be disengaged immediately during brake actuation in a coasting state), causing the lock-up clutch to move to a disengagement side (a side with a large amount of slip, where the clutch tends to slip). Because of this, even if the transmission ratio is increased, it becomes difficult to transmit torque from the drive wheel to the engine (in a fuel injection halting state). The engine speed will thus not increase, which may cause engine stall. As such, when learning is incomplete, the differential pressure of the lock-up clutch in a coasting state cannot be set low (cannot be set to allow immediate disengagement). As a result, the lock-up clutch cannot be disengaged immediately when a brake is applied in a coasting state, resulting in reduction in engine speed and possibly engine stall.
Even if an immediate speed change toward a larger transmission ratio can be effected and the engine can be maintained in a sufficient driven state, deceleration acting on the vehicle changes rapidly, which gives uncomfortable feeling to travelers on the vehicle.